A new method for mapping Earths interior shows broad regions under the Pacific
Ocean and Africa where magma hotter than its surroundings is floating through
the upper mantle and then traveling horizontally under Earths lithosphere.
Some seismologists have called these slow upwelling regions superplumes, but primarily
relegated the phenomena to the lower mantle.

Superplumes refer
to these two features at the bottom of the mantle, not the hot spot surface expression
of narrow plumes, says Barbara Romanowicz of the University of California,
Berkeley. Seismologists dont know yet if these superplumes are large
features from one plume or a collection of much more narrow plumes in the sense
that geodynamicists think of them.

In the April 19 Science, Romanowicz suggests that the continuation of the
superplumes through the mantle may require reevaluating Earths heat budget
from the core and the role upwelling from the mantle plays in driving plate tectonics.

The top map shows seismic velocity in the
lower mantle. The bottom map shows the attenution (a decrease in amplitude or
energy) of seismic waves in the upper mantle. High attenuation occurs in areas
of high temperature. Image supplied courtesy of Romanowicz and Gung.

In these broad regions of extra hot magma, seismic waves traveling through the
mantle slow down. While previous studies indicated these superplumes traveled
high above the core-mantle boundary, other models of the upper mantle dont
show as clearly the relationships between these two superplumes in the lower and
upper mantle, Romanowicz says.

Rather than using the changing velocity of seismic waves to determine the temperature
patterns of the upper mantle, Romanowicz and graduate student Yuancheng Gung studied
the amplitudes of these waves. Seismic waves travel faster through cold
and rigid slabs, but travel time  or velocity  is sensitive to chemical
composition and in the upper mantle the effects are competing, Romanowicz
says. Temperature distribution blurs travel time signals, making it more
difficult to interpret.

They produced 3-D maps that showed decreasing amplitudes of the waves, or their
attenuation through hotter-than-average material in the upper mantle. Were
using attenuation measurements that are more sensitive to temperature. It measures
how seismic waves lose energy as they propagate through hot, soft absorbing material,
Romanowicz explains.

Linking their maps to previous velocity maps of the lower mantle created a close
match. They show the hotter bits in the lower mantle are beneath the hotter
bits in the upper mantle. One is above the other, says geologist Kevin Burke
of the University of Houston. But at some points the flow has shifted in the upper
mantle, indicating a change in direction of the upwelling flow. This is
an innovative, comprehensive review on the way in which heat moves in the mantle-driving
forces for plate tectonics, he says.

But the role these hot areas play in perpetuating the cycle of plate tectonics
is itself a contentious sticking point for modelers. Geodynamicists say
the most important feature in mantle-driving convection pattern is the cold slabs
falling into the mantle and displacing material, Romanowicz says. They
consider the return flow that rises a passive return flow. We think because these
features are strong and continue through the mantle that they must be putting
energy into the system. Some heat from the core is basically driving the upwellings.

Gerald Schubert of the University of California, Los Angeles is skeptical of Romanowiczs
findings, calling the comparison between the attenuation of the upper mantle and
the velocity of the lower mantle tenuous. Because seismic velocity measurements
come with differing interpretations, a better comparison would be attenuation
throughout the mantle and show the connection between the upper and lower mantle
this way, he says. Then you could significantly say when you measure
attenuation you have a stronger case to interpret measurement in terms of temperature.